1. Field of the Invention
The present invention relates to the field of apparatus and methods for detecting anomalies in ferrous pipe structures, such as natural gas or oil pipelines, through the use of magnetic sensors which are passed along the interior of the pipe structures.
2. The Prior Art
Piping systems, such as natural gas or oil piping systems, need to be inspected, after construction, and periodically after use has begun, for the purposes of detecting defects or points of failure or leakage, or in some cases, predicting such points of failure or leakage. Since such pipelines are typically buried or submerged, it has become necessary to develop a means for inspecting such pipe structures using preprogrammed robotic or remotely operated or teleoperated devices.
It is known, for example, that in ferrous pipe structures, such as are used for natural gas or petroleum, the pipe will have a residual or remnant magnetic field associated with it, which can be detected and measured by appropriate electromagnetic sensors placed next to or against the pipe structure. It is also known that by placing magnetic field sources next to such pipe structures, a portion of the magnetic flux from the sources can be forced to travel through the pipe structure. When sensors are activated to seek the imposed field(s) passing through a pipe structure, if the particular local section of pipe is without welds, flaws or other anomalies, then the imposed fields will not be detected. If, however, the local section of pipe has welds, cracks, or other flaws or anomalies, then the imposed field will "leak" from the anomaly and be detected by the sensors, when the leakage field is compared to the profile of the residual or remnant field detected and measured for the same local portion of pipe.
Numerous examples of anomaly detection devices, using magnetic field generators and magnetic field sensor, exist in the prior art, including such devices as are disclosed in Beaver et al., U.S. Pat. No. 3,460,028; Barton, U.S. Pat. No. 3,593,122; Smith, U.S. Pat. No. 4,105,972; Birchak et al., U.S. Pat. No. 4,649,343; Ando et al., U.S. Pat. No. 4,742,298; and Cecco et al., U.S. Pat. No. 4,855,676.
The Beaver et al. '028 reference shows an anomaly detector having a single axially oriented magnetic field generating apparatus centrally arranged within the sensor rig. Annular brushes positioned fore and aft of a plurality of magnetic field sensor "sledges" on the sensor rig convey the magnetic field into and out of the pipe structure, such that the magnetic flux lines are parallel to the longitudinal axis of the pipe structure. The "sledges" are held in spring-biased relation against the interior surface of the pipe structure, as the sensor rig moves along the interior of the pipe structure.
The apparatus of the Barton '122 patent likewise employs fore and aft magnetic pole pieces to establish an axially extending magnetic flux path, with sensor "shoes" positioned axially between the pole pieces. An additional pole piece, positioned aft of the paired pole pieces, and having brush parts inclined relative to the interior surface of the pipe structure acts to strengthen the residual magnetic field in the pipe structure. The residual fields detected are compared to the readings taken when the pipe structure has the active magnetic field imposed upon it. The discrepancies in the two sets of readings indicates the presence of anomalies, which may be welds, or actual flaws in the pipe structure.
The Smith '972 patent shows a pipeline inspection vehicle having a plurality of sensor heads arranged in a circle about the vehicle, and held in place against the interior surface of the pipe structure by an annular, spring-biased structure. Each individual sensor head may have magnetic field generating apparatus therein, as well as magnetic field sensing apparatus.
The Birchak et al. '343 patent describes an inspection system for use in small bore tubes, in which a scanner body has two annular magnetic cores, arranged perpendicular to one another, inside a hollow core of the scanner body. An array of magnetic field sensors are arranged circumferentially around the scanner body. The field generated by the two magnetic cores, simultaneously, may be manipulated by phase shifting and amplitude variations, so as to shift the direction of the field, even to producing a helical magnetic flux, or to swing the flux through nearly 180.degree. to expose an anomaly to magnetic flux directed normal to it.
The apparatus of the Ando et al '298 patent employs an axially extending primary magnetic coil, and a plurality of circumferentially extending secondary coils positioned radially outwardly of the primary coil. The secondary coils do not impose a magnetic field, but rather sense the axial component of the magnetic flux generated by the primary coil, in the form of a voltage imposed on the secondary coil. The sensed component changes in the presence of a flaw positioned between the poles of the primary coil.
The Cecco et al. '676 reference shows an apparatus for inspection of a pipe, having a sensor member configured to produce both axially extending and radially extending magnetic fields, positioned along the length of the sensor member. The Cecco et al. reference describes that both fields are of such strength as to obtain very high levels of saturation in the pipe structure.
The present invention is particularly directed to inspection systems for ferrous pipe structures, which are particularly suited to small bore pipe structures, such as the piping systems for natural gas distribution, the pipes of which typically have a nominal four-inch interior diameter. Commonly, prior art defect detection systems, which have typically been configured for much larger diameter pipes, have relied upon detection techniques involving the complete saturation of the pipe structure by the generated magnetic fields. Such saturation involves substantial power consumption by electro-magnets, or the use of large, typically heavy, permanent magnets.
To provide support, such as power cables and transformers or sufficiently powerful and durable portable power supplies for electromagnets, and/or conveyance mechanisms for the sensors for saturation-type detector devices, in the confined environment of a small-bore detector device, is a difficult task. It is desirable, therefore, to provide a way of avoiding the logistical difficulties presented by saturation-type detector systems.
Particular problems are presented by anomalies which are closer to the outside surface of the pipe structure, such as cracks beginning from the outside, toward the interior of the pipe. As mentioned, due to space requirements, the power of the magnetic field generating means which can be carried on board the sensor is limited. Further, while the magnetometers which are carried on the sensor, are often carried in shoes which are held in a spring or mechanically biased manner against the inner surface of the pipe structure, there will still be an "air-gap" between the surface of the shoe and the pipe interior surface, across which the magnetic fields must jump to reach the pipe from the generators and return to the magnetometers. This gap, which may be only a few thousandths of an inch, is still enough to cause substantial attrition in the strength of the field being imposed upon the pipe, and subsequently being detected by the magnetometers.
An even more important potential problem is that the interior surfaces of gas pipes can be relatively irregular, with numerous bumps and ridges. This irregular surface forms part of the magnetic field circuit, causing variations in the emitted and sensed magnetic field. The irregularities thus become a source of error in the observed fluctuations of the magnetic field, as the sensor shoes pass over the interior surface of the pipe.
It is desirable, therefore, to provide a way of imposing a magnetic field into a pipe structure, without requiring large or complicated magnetic field generating devices to be carried by the sensor.
It is also desirable to provide a way of imposing a magnetic field into a pipe structure, which is suitable for use with sensors which are sized for small pipe diameter applications.